Recently, central processing units (CPUs) have to bear highly dynamic load currents that usually change very quickly from a light load to a maximal-load. The CPU current transient may happen within 1 μs, smaller than a switching cycle of a typical PWM voltage regulator, whether it is controlled in a voltage mode or a current mode. For solving this problem, a PWM voltage regulator that serves to provide a voltage to a CPU is usually set with a threshold of output voltage variation so that when the variation of its output reaches the threshold, another non-closed loop adjusting mechanism can be triggered. For example, another PWM on-time can be triggered or its off-time can be immediately stopped or the duty of a PWM signal can be increased. However, such an approach has two major problems. First, the threshold of voltage variation is discrete, so transient response can only be improved when the droop-voltage at the output exceeds the threshold. Second, the threshold of voltage variation is fixed, and thus it can't meet a variety of applications. Additionally, in the event that the threshold setting relies on external components, additional pins will be required, which increases manufacturing costs and reduce the flexibility of circuit design.
Currently, voltage regulators for CPUs, for example U.S. Pat. No. 7,436,158, mostly use native adaptive voltage positioning (N-AVP) control. Conventional PWM structures usually use a ramp signal as the reference to be compared with the output voltage or the inductor current for generating PWM signals to control switching of voltage regulators. During transient where the load changes from a light to a heavy, the output voltage of a PWM voltage regulator drops suddenly, and this may lead to shutdown of the CPU. For improving control loop transient, there have been proposed many solutions. For example, U.S. Pat. Application publication No. 20070013356 uses a voltage-mode control loop to achieve quick transient response, while it suffers a timing issue caused by a synchronous clock and is unable to act instantly when transient occurs, U.S. Pat. Application Publication No. 20070109825 changes timing sources by detecting a load current. Although this art is helpful to solve the foregoing problem about clock timing, it is also unable to act instantly when transient occurs. U.S. Pat. No. 7,615,982 inserts a non-closed loop PWM pulse when the load current exceeds a preset threshold to improve transient response. While this art realizes instant transient response, its non-linear control can undesirably make the control loop of the voltage regulator unstable.